Power management mechanism for loop powered time of flight and level measurement systems

ABSTRACT

A loop powered level measurement and time of flight ranging system with power management. The level measurement system comprises a controller, a transducer for distance ranging, a power supply, and a power management stage. The power management stage comprises a shunt current regulator and a circuit for charging a storage capacitor. The shunt current regulator shunts excess current from the current loop and the circuit charges the storage capacitor. The charging of the capacitor is monitored and stored energy is applied to the transducer to make a level measurement. During charging, the controller continues to operate a user interface and communication module.

RELATED APPLICATION

[0001] This application claims the benefit under 35 U.S.C. §119 offoreign patent application serial no. 2,406,298 filed on Sep. 30, 2002in Canada, which is incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to time of flight ranging systemsand level measurement systems, and more particularly to a powermanagement mechanism and technique for loop powered level measurementsystems.

BACKGROUND OF THE INVENTION

[0003] Loop powered level measurement systems operate on a 4-20 mAcurrent loop, hence the name loop powered. The circuitry for the levelmeasurement system, i.e. the load, is typically designed to operate atless than 4 mA. The current loop provides a terminal voltage in therange 12-30V, but is nominally 24V.

[0004] To take a measurement, power is applied to the transducer and thereflected pulses are detected and the distance to the reflective surfaceis calculated or measured. If more than 4 mA is needed to make ameasurement, then energy taken from the current loop is stored untilthere is enough to make the measurement. In addition, to make rapidmeasurements, more current from the loop is also needed. As the currentin the loop increases, the speed of measurement also increases. Sincethe power available from the current loop is less than the powerrequired to continuously operate the level measurement device, the levelmeasurement device is operated intermittently. In a typical levelmeasurement system, measurements are taken once every second up to onceevery five seconds.

[0005] Accordingly, there remains a need for power management in thefield of loop powered level measurement or time of flight rangingsystems.

BRIEF SUMMARY OF THE INVENTION

[0006] The present invention provides a mechanism and method for powermanagement in a level measurement or time of flight ranging system.

[0007] In a first aspect, the present invention provides a levelmeasurement system, the level measurement system is powered by a twowire loop, the level measurement system comprises: (a) a transducer foremitting energy pulses and detecting reflected energy pulses; (b) acontroller having a component for controlling the transducer, and acomponent for determining a level measurement based on the time offlight of the reflected energy pulse; (c) a power supply having an inputport for receiving power from the loop, and a component for producing anoutput voltage; (d) a power management unit coupled to the loop, thepower management unit having an output coupled to a storage capacitorfor charging the storage capacitor, an input port for receiving excesspower from the loop; (e) the transducer including an input for receivingenergy from the storage capacitor under the control of the controller.

[0008] In another aspect, the present invention provides a levelmeasurement system, the level measurement system is powered by a currentloop, the level measurement system comprises: (a) a transducer foremitting energy pulses and detecting reflected energy pulses; (b) acontroller with a component for controlling the transducer, and acomponent for determining a level measurement based on the time offlight of the reflected energy pulse; (c) a power supply having an inputport coupled the current loop for receiving current at a voltage level,and the power supply having a component for producing an output voltagefor powering the controller; (d) a power management unit coupled to thecurrent loop, the power management unit having an output coupled to astorage capacitor for charging the storage capacitor, an input port forreceiving excess current from the current loop; (e) the transducerincludes an input for receiving energy from the storage capacitor underthe control of the controller.

[0009] Other aspects and features of the present invention will becomeapparent to those ordinarily skilled in the art upon review of thefollowing description of specific embodiments of the invention inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Reference is next made to the accompanying drawings, which show,by way of example, embodiments of the present invention and in which:

[0011]FIG. 1 shows in diagrammatic form a loop powered level measurementsystem and power management mechanism according to the presentinvention.

[0012]FIG. 2 shows in block diagram form the loop powered levelmeasurement system according to the present invention;

[0013]FIG. 3 shows in schematic form an implementation for the powermanagement circuitry the level measurement system in FIG. 2.;

[0014]FIG. 4 shows in schematic form a circuit implementation for thewaste power supply module; and

[0015]FIG. 5 shows in schematic form a circuit implementation for theshunt current regulator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] Reference is first made to FIG. 1 which shows a loop poweredlevel measurement system with power management according to the presentinvention. The loop powered level measurement system, indicatedgenerally by reference 10, interfaces to a power/communication loop 20,for example a 4-20 mA current loop. The loop powered level measurementsystem 10 is coupled to the current loop 20 at terminals A and B. Aremote receiver, for example a plant control computer, indicated byreference 8 is coupled at the other end of the current loop 20. For atypical 4-20 mA current loop configuration, the loop 20 provides acurrent in the range 4 to 20 mA and a loop voltage in the range 18 to 30Volts. The loop voltage is nominally at 24 Volts and represented as avoltage source with reference 22. The resistance of the loop isrepresented as a resistive element 24 and is typically in the range 0 to550 Ohms. While the loop current is normally in the range 4 to 20 mA,the current may range from 3.6 to 21.6 mA to indicate alarm conditions.

[0017] Reference is next made to FIG. 2 which shows in more detail thelevel measurement system with power management 10 according to theinvention. The level measurement system 10 comprises a transducer module101, a controller 102, a power supply 104, a shunt current regulator106, a loop current sensor and amplifier circuit 108, a power managementmodule 110 and an energy storage capacitor 112. The level measurementsystem 10 may also include a communication module 114.

[0018] The power supply 104 comprises a switching power supply and isdesigned to consume less than the minimum loop current, nominally 4 mA.

[0019] The shunt current regulator 106 is operated under firmwarecontrol by the controller 102 to draw additional current to achieve thedesired current in the current loop 20. One terminal of the shuntcurrent regulator 106 is connected to terminal A of the current loop 20and the input to the power supply 104. The other terminal of the shuntcurrent regulator 106 is coupled to the power management module 110. Theshunt current regulator 106 has a control terminal which is coupled to acontrol output port on the controller 102. The shunt current regulator106 is also used by the power management module 110 to charge thestorage capacitor 112 as described in more detail below. The loopcurrent sensor and amplifier circuit 108 senses the current flowing inthe loop 20 and provides feedback for the shunt current regulator 106and the controller 102. The loop current sensor and amplifier circuit108 may be implemented with a current sensing resistor and an amplifieras shown in FIG. 4.

[0020] As shown in FIG. 2, the level measurement system 10 also includesa user interface module 116. The user interface module 116 comprises adisplay, for example, a LCD module, and a keypad or touch sensitiveoverlay on the LCD.

[0021] The transducer module 101 is coupled to a control port andinput/output port on the controller 102. The transducer module 101includes a transducer, a transmitter stage and a receiver stage (notshown). The transducer (not shown) may comprise radar-based technology,ultrasonic-based technology, TDR-based technology (time domainreflective), or other distance ranging technology. Under the control ofa program stored in memory (e.g. firmware), the controller 102 generatesa transmit pulse control signal for the transmit stage in the transducermodule 101, and the transducer (not shown) emits a transmit burst ofenergy, for example, radar pulses directed at the surface of a material50 (FIG. 1) contained in a storage vessel 60 (FIG. 1). The reflected orecho pulses, i.e. the propagated transmit pulses reflected by thesurface 52 of the material 50 (FIG. 1), are coupled by the transducer,for example, a radar antenna or other distance ranging technology (notshown), in the transducer module 101 and converted into electricalsignals by the receiver stage (not shown). The electrical signals areinputted by the controller 102 and sampled and digitized by an A/Dconverter (not shown) and a receive echo waveform or profile isgenerated. The controller 101 executes an algorithm which identifies andverifies the echo pulse and calculates the range, i.e. the distance tothe reflective surface, from the time it takes for the reflected energypulse to travel from the reflective surface 52 (FIG. 1) to thetransducer (not shown) in the transducer module 101. From thiscalculation, the distance to the surface of the material 50 and therebythe level of the material 50 in the vessel 60 is determined. Thecontroller 101 may comprise a microprocessor or a microcontroller withon-chip resources, such as an A/D converter, ROM (EPROM), RAM. Themicroprocessor or microcontroller is suitably programmed to performthese operations as will be within the understanding of those skilled inthe art.

[0022] Referring to FIG. 2, all power for the operation of the levelmeasurement system 10 is derived from the current loop 20. The powersupply module 104 comprises a switching power supply which takes itspower input from the current loop 20 and generates the appropriatevoltage levels, e.g. supply rails, for the circuitry, i.e. thecontroller 102, the display and user interface module 106 and the otherelectronic modules in the level measurement system 10. The transducermodule 101 is powered from the waste power supply 110 as described inmore detail below.

[0023] The controller module 102 also controls the transmission of dataand control signals through the interface with the current loop 20. Thecontroller 102 uses the shunt current regulator 106 to adjust ormodulate the loop current in the range 4 to 20 mA to transmit thecalculated level of the material 50 to the remote receiver or plantcomputer 8 (FIG. 1) connected to the other end of the current loop 20(FIG. 1). As shown in FIG. 2, the level measurement system 10 mayinclude the communication module 114. The communication module 114includes a digital communication modem, for example a HART modem, whichprovides another communication channel between the controller 102 andthe remote computer 8 (FIG. 1) over the wires of the current loop 20.

[0024] In operation, the user interface module 116 comprising thedisplay module and the keypad, and the digital communication module 114are run continuously. The display, user interface and communicationoperations may be thought of as primary functions which runcontinuously. The transducer module 101 is operated intermittently totransmit energy pulses and detect reflected energy pulses from thesurface of the material 50 contained in the vessel 60.

[0025] The power available from the current loop 20 for the levelmeasurement system 10 is given by:

(loop voltage−(loop current×loop resistance))×loop current

[0026] In order to achieve the fastest operation rate, all availablepower from the current loop 20 is utilized by the level measurementsystem 10.

[0027] The power management module 110 functions to tap excess power,e.g. loop current>main power supply 104, from the current loop 20, aswill be described in more detail below. As shown in FIG. 2, the powermanagement module 110 is coupled to the storage capacitor 112 and theshunt current regulator 102.

[0028] The power management module 110 uses the shunt current regulator102 to charge the storage capacitor 112. The power supply 104 isdesigned to operate at less than the lower current loop limit, forexample, 3 mA, which means that at least 1 mA of loop current is shuntedby the shunt current regulator 106 and to the power management module110 to charge the storage capacitor 112. As shown, the controller 102has an input port coupled to the storage capacitor 112. The controller102 senses the voltage on the storage capacitor 112. When the voltagelevel on the capacitor 112 is sufficient to power the transducer module101, the controller 102 activates the transducer 101 to perform a levelmeasurement for the vessel 60 (FIG. 1). The controller 102 thendeactivates the transducer module 101 and the capacitor 112 is allowedto recharge using the power management module 110. The level measurementcalculated by the controller 102 is transmitted to the remote computer 8(FIG. 1) as a communication task for the primary functions. It is notnecessary to turn off the controller 102 between measurements, and thecontroller 102 continues to execute the refresh operation and keypadscan functions for the user interface module 116, and the communicationfunction.

[0029] Reference is next made to FIG. 3, which shows in schematic form acircuit implementation for the waste power supply module 110. As shownin FIG. 3, the waste power supply 110 comprises a regulator circuit 202and a clamp circuit 204. In the waste power supply 110, terminal 206(PRAW) is coupled to the shunt regulator 106 (FIG. 2) and the storagecapacitor 112 (FIG. 2). Terminals 208 (COM) are connected to the commonrail (COM) for the system 10. One terminal of the loop current sensor108 (FIG. 2, FIG. 4). Terminal 210 (CLAMP*) is coupled to the controller102 (FIG. 2), and driven by the controller 102 during start-up. Terminal212 (PHV) is coupled to terminal A (FIG. 1).

[0030] In operation, the regulator circuit 202 tries to maintain PRAW,i.e. terminal 206, approximately 2 Volts below PHV, i.e. terminal 212.As the voltage on PRAW drops to 2 Volts less than the voltage on PHV,diodes D10 and D11 begin to turn off, which causes transistors Q9 and Q6to turn. With the transistors Q6 and Q9 on, the impedance between PRAWand COM_CAP is reduced. This tends to increase the voltage differencebetween PRAW and PHV, thereby maintaining an approximate 2 Voltdifference between PRAW and PHV.

[0031] In the operation of the clamp circuit 204, when the terminal 210,i.e. CLAMP*, is LOW, resistors R31 to R36 connected in series with diodeD13 are coupled across terminal A and the common rail COM. This providesa load while the controller 102 is starting up or initializing. Theterminal 210 or CLAMP* is maintained low during start up, and whenCLAMP* is high the load from resistors R31-R36 is disconnected. Thisload on start up is provided to ensure a current draw greater than 22.6mA from the current loop 20.

[0032] Reference is next made to FIG. 4 which shows a circuitimplementation for the loop current sensor and amplifier circuit 108.The loop current sensor and amplifier circuit 108 comprises a resistor220 and an operational amplifier circuit 222. The loop current sensor108 has terminal 224 (NEG), terminal 226 (COM), and output terminal 228(LOOP_SENSE). The NEG terminal 224 is coupled to terminal B (FIG. 2) andthe current loop 20 (FIG. 1). The COM terminal 226 is connected to thecommon or return supply rail. The LOOP_SENSE terminal 228 provides theoutput for the loop current sensor 108 and is formed from the output ofthe operational amplifier in the circuit 222. The LOOP_SENSE terminal iscoupled to an input on the shunt current regulator 106 as shown in FIG.5.

[0033] Reference is next made to FIG. 5, which shows a circuitimplementation for the shunt current regulator 106. The shunt currentregulator 106 as shown in FIG. 5 comprises a Pulse Width Modulator (PWM)circuit 230, an error amplifier circuit 232, a controlled current source234, and a filter and voltage drop circuit 236.

[0034] The PWM filter circuit 230 has an input terminal (PWM) 240, andan output terminal (ANALOG_OUT) 242. The PWM terminal 240 is coupled toan output port on the controller 102 (FIG. 2) which is used to set thecurrent level, i.e. between 4 to 20 mA. The ANALOG_OUT terminal 242 iscoupled to the “Modulate” signal port on the HART modem 114 (FIG. 2). Inoperation, the PWM filter circuit 230 low pass filters the PWM signal onthe input terminal 240 to produce a DC voltage, i.e. PWM output, whichis proportional to the duty ratio or duty cycle of the PWM signalreceived from the controller 102 (FIG. 2).

[0035] The error amplifier circuit 232 has an input terminal(CURRENT_SENSE) 244, terminal (PHV) 246, terminal (PRAW) 248, andterminal (PHFV) 250. The CURRENT_SENSE input terminal 244 is coupled tothe LOOP_SENSE output terminal 228 (FIG. 4) from the loop current sensor108. The PHV terminal 246 is coupled to the terminal A (FIG. 2). Theerror amplifier circuit 232 amplifies the difference between the actualcurrent, i.e. as indicated by the input CURRENT_SENSE 244, and therequested current, i.e. the PWM output from the PWM filter circuit 230which is generated from the input PWM 240. This difference is used inthe feedback loop to control the loop current.

[0036] The controlled current source circuit 234 is coupled to theoutput of the error amplifier 232. The controlled current source 234 isconnected to the PHV terminal 246. The controlled current source 234also includes terminal (PRAW) 248. The PRAW terminal 248 connects to thecorresponding PRAW terminal 206 in the waste power supply 110 (FIG. 3).As shown the controlled current source circuit 234 includes a number ofJFET transistors 254, indicated individually as 254 a, 254 b, 254 c, 254d, 254 e which are coupled in parallel. The parallel arrangement of theJFET transistors 254 allows for higher saturation currents.

[0037] The filter and voltage drop circuit 236 is coupled to terminalPHV 246 and includes a BIAS terminal 250 and a terminal (PHVF) 252. Asalso shown, the filter and voltage drop circuit 236 includes an inductor(L11) 256, a diode circuit 258 comprising diodes 260 a, 260 b, 260 c,260 d, and a capacitor (C65) 262. The arrangement of the inductor 256and the capacitor 262 filters noise arising from the power supply 104from feeding back in the current loop. The diode circuit 258 is providedfor clamping the inductor 256 for intrinsically safe, i.e. I.S.,applications as required. The remainder of the circuit 236 as shown inFIG. 5 provides a voltage drop of approximately 0.7 Volts between thePHV terminal 246 and the PHVF terminal 252 as required for operation ofthe HART modem 114 (FIG. 2).

[0038] In operation, the less current the main power supply 104 drawsfrom the loop 20, the more loop current flows to the shunt currentregulator 106 and to the power management module 110 for faster chargingof the storage capacitor 112. The faster the charging of the capacitor112, the shorter the level measurement cycle. To improve chargingefficiency, the power supply 104 is implemented as a switching powersupply in the level measurement system 10. This means that as the loopvoltage increases, the loop current drawn by the power supply 104decreases resulting in an increase in the loop current to the shuntcurrent regulator 106 and the power management module 110 for fastercharging of the storage capacitor 112.

[0039] To further improve the charging efficiency of the storagecapacitor 112, the microprocessor or microcontroller for the controller102 may be operated at a slower clock speed, e.g. under control of thefirmware and the instruction set specific to the microprocessor device.The microprocessor requires less current when operating at a slowerclock speed. The primary functions, such as display, user interface andcommunications, can be performed at the slower clock speeds.

[0040] The present invention may be embodied in other specific formswithout departing from the spirit or essential characteristics thereof.Certain adaptations and modifications of the invention will be obviousto those skilled in the art. Therefore, the presently discussedembodiments are considered to be illustrative and not restrictive, thescope of the invention being indicated by the appended claims ratherthan the foregoing description, and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced therein.

What is claimed is:
 1. A level measurement system, said levelmeasurement system being powered by a two wire loop, said levelmeasurement system comprising: (a) a transducer for emitting energypulses and detecting reflected energy pulses; (b) a controller having acomponent for controlling said transducer, and a component fordetermining a level measurement based on the time of flight of saidreflected energy pulse; (c) a power supply having an input port forreceiving power from the loop, and a component for producing an outputvoltage; (d) a power management unit coupled to the loop, said powermanagement unit having an output coupled to a storage capacitor forcharging said storage capacitor, an input port for receiving excesspower from the loop, a control terminal responsive to a control outputfrom said controller for controlling the charging of said storagecapacitor; (e) said transducer including an input for receiving energyfrom said storage capacitor under the control of said controller.
 2. Thelevel measurement system as claimed in claim, wherein said powermanagement unit includes a control terminal responsive to a controloutput from said controller for controlling the charging of said storagecapacitor.
 3. The level measurement system as claimed in claim 1 or 2,further including a user interface module and a communication module fortransmitting level measurement data over the two wire loop.
 4. The levelmeasurement system as claimed in claim 3, wherein said controlleroperates said transducer intermittently and based on the charging ofsaid storage capacitor.
 5. The level measurement system as claimed inclaim 4, wherein said controller operates said user interface module andsaid communication module continuously.
 6. The level measurement systemas claimed in claim 5, wherein said power supply comprises a switchingpower supply, and wherein the two wire loop comprises a current loopoperable between a minimum current level and a maximum current level,and said switching power supply operating at current level less thansaid minimum current level, and the difference in the current levelsbeing directed to said power management unit for charging said storagecapacitor.
 7. A level measurement system, said level measurement systembeing powered by a current loop, said level measurement systemcomprising: (a) a transducer for emitting energy pulses and detectingreflected energy pulses; (b) a controller having a component forcontrolling said transducer, and a component for determining a levelmeasurement based on the time of flight of said reflected energy pulse;(c) a power supply having an input port coupled to the current loop forreceiving current at a voltage level, and said power supply having acomponent for producing an output voltage for powering said controller;(d) a power management unit coupled to the current loop, said powermanagement unit having an output coupled to a storage capacitor forcharging said storage capacitor, an input port for receiving excesscurrent from the current loop; (e) said transducer including an inputfor receiving energy from said storage capacitor under the control ofsaid controller.
 8. The level measurement system as claimed in claim 7,wherein said power management unit includes a control terminalresponsive to a control output from said controller for controlling thecharging of said storage capacitor.
 9. The level measurement system asclaimed in claim 7 or 8, wherein said power supply comprises a switchingpower supply and said switching power supply operates at a current levelbelow the operating current level for the current loop, and said powermanagement unit utilizes the difference in said current levels forcharging said storage capacitor under the control of said controller.10. The level measurement system as claimed in claim 9, furtherincluding a user interface module and a communication module fortransmitting level measurement data over the current loop.
 11. The levelmeasurement system as claimed in claim 10, wherein said controlleroperates said transducer intermittently and based on the charging ofsaid storage capacitor.
 12. The level measurement system as claimed inclaim 11, wherein said controller operates said user interface moduleand said communication module continuously.
 13. The level measurementsystem as claimed in claim 12, wherein said power management unitcomprises a shunt current regulator coupled to the current loop, andhaving control terminal responsive to a control output from saidcontroller for shunting current from the current loop.
 14. The levelmeasurement system as claimed in claim 13, wherein said power managementunit comprises a current sensor coupled to the current loop and havingan output for indicating the current level in the current loop.